Can Plant - Herbivore Interaction be affected by Selenium?

Review Article

Austin J Environ Toxicol. 2015;1(1): 5.

Can Plant - Herbivore Interaction be affected by Selenium?

Mechora Š1* and Ugrinović K2

1Department of biology, University of Maribor, Slovenia

2Agricultural Institute of Slovenia, Slovenia

*Corresponding author: Mechora Š, Department of biology, University of Maribor, Koroška cesta 160, 2000 Maribor, Sloveni

Received: September 03, 2014; Accepted: September 17, 2014; Published: February 12, 2015


Selenium acts as an antioxidant, therefore in low concentrations increase the plant tolerance to drought, salinity, UV induced stress and metals. All these can be beneficial for the growth of plants and can positively affect their yield. In higher concentrations it can be toxic, but some plants can tolerate elevated concentrations. These are called Se accumulators. High selenium tissue concentration may affect other species, such are herbivores. Previous studies demonstrated that insects do accumulate selenium, but the effects of this element on insect growth and survival is limited. Laboratory studies have shown that selenate incorporated into the diet affects feeding site preferences and host plant selection, reduce growth and acts as antifeedant for larvae of the generalist herbivory beet armyworm (Spodoptera exigua). Selenium accumulation in tissues protect plants against caterpillars of cabbage looper (Trichoplusia ni), larvae of the cabbage white butterfly (Pieris rapae), green peach aphids (Myzus persicae), nematodes and prairie dogs. Therefore selenium can reduce herbivore's growth, cause toxic effects or deter herbivores, and at the same time positively affects plant growth.

Keywords: Selenium; Se accumulator; Herbivore; Pest; Nematode


Selenium (Se) is a trace element, which is essential for humans and animals, while its essentiality for higher plants has not yet been proven. Many plant species grow on soils with moderate concentrations of Se, while some can tolerate high concentrations and even accumulate it. High Se concentrations occur naturally in some soils, especially those derived from Cretaceous shale parent material. Se content of soils ranges from almost zero up to 1250 mg Se kg-1 in some seleniferous soils in Ireland [1], but Se accumulator plants can tolerate elevated concentrations and even need it for their growth [2]. Se accumulators are predominantly found on Se-rich soils as seen in the western United States [3]. Many hypotheses for the functional significance of metalloid accumulation by plants have been proposed, including allelopathy, drought resistance and protection from both herbivores and pathogens [4]. The latter hypothesis is termed the elemental defense hypothesis and many studies confirmed it.

Higher concentrations of Se can be toxic for organisms. Se pollution of soil and water is a major environmental problem in many areas of the world [5]. Se can leak in the environment from soil with the irrigation of crops. Other anthropogenic activities also lead to Se pollution of water. The oxidized forms of Se, selenite and selenate, are the most soluble and predominate in natural water systems. Because of their high solubility, these forms are more available for plants [6] and are potentially toxic to organisms. Once in the aquatic environment, Se can rapidly attain levels that are toxic to wildlife because of its bioaccumulation in food chains and the resulting dietary exposure [7]. Aquatic insect larvae, as a significant component of the benthos, can be important carriers of trace elements (including Se) from sediment into the water column and to the terrestrial food chains.

Insects have great importance in most ecosystems and a significant contribution to the biological cycling of trace elements. Se accumulator plants obtain high levels of this element and since they are a part of the food chain, it is possible that Se in the tissue affect herbivores. This paper gives first a short background on the Se and its effects on plants and then brings a detailed overview of studies dealing with the Se effect on herbivores.

Selenium - the powerful versatile trace element

Se was discovered in 1817 by the Swedish chemists Jöns Jakob Berzelius. The name for this trace element comes from the Greek word for moon - selene. Se is one of metalloids, which is found in the Earth's crust, soils, minerals, in freshwater, seawater, and in sediments. It is present in organic and inorganic compounds in four oxidation states - elemental (Se0), selenite (Se+4), selenate (Se+6) and selenide (Se-2) [8], while in nature it is found in various volatile (dimethylselenide, dimethyldiselenide) and other organic compounds (SeMet, SeCys, etc.) [9]. The form of Se present in a given system depends on soil properties, including pH, redox potential, salinity and the content of calcium carbonate (CaCO3) [10]. Se is unevenly distributed over the globe. Some parts of the world contain deficit Se concentrations of 0.01 mg kg-1 at the Russian Plane to heavily toxic values of 1200 mg kg-1 in organic soils at Meath, Ireland [11]. In acid soils it is mainly present in the form of selenite, while in alkaline soils it is present in the form of selenate, which is more soluble and more available for uptake [12].

The perception of the element evolved since its discovery in 19th century. At first, Se was known as toxic and carcinogenic element. This perception changed when Schwarz and Foltz discovered its essentiality for humans and animals in 1957 [13]. Since then it is used in industry, pharmacy as well as in agronomy. Agricultural applications of Se include an additive and dietary supplement in animal feeds. In part of the world where soils are deficient with Se, the addition of Se to fertilizers and top dressings [14] is a good way to improve Se status. In Finland, Se-enriched fertilizers have been used in plant production since 1984 to increase Se content in agricultural plants [15].

The margin between Se essentiality and toxicity is rather narrow. At higher concentrations, Se can be toxic due to its chemical similarity to sulfur (S), leading to nonspecific replacement of S by Se in proteins, causing toxicity [16]. Still today, chronic or acute Se poisoning is responsible for the loss of thousands of head of livestock every year in the western United States [3], where Se is prevalent in soils. Selenosis in animals mostly is due to ingestion of Se accumulator plants. There are many studies regarding Se toxicity in farming animals, but only few investigated the effect of Se on invertebrate herbivores.

Beneficial effects of selenium on plants

Se has been found an essential element for green algae Chlamydomonas reinhardtii [17]. For higher plants, it has not yet been shown to be an essential but it is considered a beneficial nutrient [18]. In plants, it functions as an antioxidant [19].

Many plant species grow on soils with moderate concentrations of Se, while some plants can tolerate high concentrations and even accumulate it. These plants are so called accumulators of Se, among which are Stanleya pinnata (Brassicaceae) and Astragalus bisulcatus (Fabaceae). They can accumulate from hundreds to several thousand milligrams of Se kg-1 dry weight [2]. Primary accumulators accumulate more than 1000 mg Se kg-1 dry weight, while secondary Se accumulators accumulate up to 1000 mg Se kg-1 dry weight [2], among which is Brassica g-1 (Brassicaceae). Non accumulator plants are able to concentrate toxic selenate and Se proteins as a result of fortification with Se [20], but to a lower extent as Se accumulators. Especially plants containing a lot of S can accumulate considerable amounts of Se without suffering substantial damage. Plants from Brassicaceae family are known to be rich in S, therefore they can absorb greater amount of Se from their surroundings. For this the family has been widely studied for supplementation of inorganic Se to cultivated plants; i.e. turnip (Brassica campestris) [21], cabbage (B. oleracea (capitata group)) [22] and broccoli (B. oleracea (italica group)) [23].

In moderate concentrations Se can increase the plant tolerance to UV-induced oxidative stress, delay senescence and promote the growth of aging seedlings [24]. It also increases plant tolerance to drought [25], salinity [26] and metals [27]. All these can contribute to higher yield of better quality. There is mounting evidence that Se can also protect plants from biotic stress.

How does selenium affect herbivores?

Although there are many studies investigating the effect of Se on plants, not many are focusing on the influence of plant accumulated Se to herbivores. The elemental defense hypothesis explains that some elements, including Se, hyper-accumulate in plants; can act as chemical defense against herbivores [4]. Several studies have confirmed this hypothesis and demonstrated that these elements can deter herbivores from feeding on plants, inhibit their growth, affect their reproduction or cause their death. Same protective effect has been confirmed also for Se (Figure 1) within the studies that have focused on the interactions between insect herbivores and Se accumulator plants, Stanleya pinnata, Brassica g-1 (Brassicaceae) and Astragalus bisulcatus (Fabaceae) [28-31]. Se accumulation has been found to have an effect on aphids, caterpillars, moths, grasshoppers, crickets, nematodes and prairie dogs [28,30-35]. Yet, Se distribution in the food chains of regions with optimal Se content in the environment as well as in the regions with Se deficiency is extremely poorly understood [20].

Citation: Mechora Š and Ugrinovic K. Can Plant - Herbivore Interaction be affected by Selenium?. Austin J Environ Toxicol. 2015;1(1): 5. ISSN:2472-372X